Electrolytic capacitors have long been known in the electronic arts. One well known electrolytic capacitor structure includes two metal foils, acting as cathode and anode, and two electrically insulating separators that allow electrical communication between the two foils by means of an electrolyte that is in contact with the foils and permeates the separator. Two separator sheets are required because the two foils and separators are spirally wound together, in the fashion of a jelly roll. The resulting cylindrical structure has a cross-section transverse to the axis of the spiral that includes the interleaved spiral layers of metal foil, separator, metal foil, and separator. This capacitor structure is easily manufactured but has limitations with regard to the density of stored energy. The second separator, which is necessary because of the winding of the foils, occupies a volume that is not occupied in, for example, a parallel plate capacitor using the two foils and only a single separator. The jelly roll structure is also limited to electrode materials that are highly flexible so that they can be wound to increase the density of stored energy.
Capacitor structures providing significantly improved density of stored energy are described in my U.S. Pat. Nos. 5,369,547, 5,469,325, 5,559,667, 5,737,181, and 5,754,394, the disclosures of which are incorporated herein by reference. In the capacitors described in my patents, a conventional anode from a wet slug capacitor is employed in combination with a cathode typically found in an electrochemical capacitor, sometimes called a pseudo-capacitive electrode.
The typical anode materials in wet slug capacitors are valve metals, i.e., metals that form an oxide naturally that may be thickened by anodic oxidation. Examples of such metals suitable for use in these anodes are tantalum, aluminum, niobium, zirconium, and titanium.
The cathodes of electrochemical capacitors are typically highly porous films that have extremely high surface areas. These cathodes may be metal oxides, nitrides, or carbides. The metals from which these oxides are formed include ruthenium, iridium, rhodium, platinum, palladium, osmium, cobalt, molybdenum, and tungsten. Suitable nitrides are made from titanium, vanadium, zirconium, nickel, molybdenum, and tungsten. Carbides usable in cathodes include vanadium, nickel, molybdenum, and tungsten carbides In addition, finely divided carbon, which has a large surface area, can function as an electrochemical cathode. As described in my prior patents, the metal oxide films are typically formed on a substrate, for example, by spraying the substrate with a compound of the metal and subsequently oxidizing the metal in the deposited film. My prior patents describe various arrangements of cathodes and anodes in capacitors employing an electrochemical cathode and a wet slug anode. The cathodes are relatively rigid in the specific cathode structures described in my prior patents and are not susceptible to winding in a jelly roll structure like the cathodes of known spirally wound electrolytic capacitors.